1. Field of the Invention
This invention relates to organosiloxane elastomer compositions in the form of a flowable powder. More particularly, this invention relates to a method for processing organosiloxane elastomer bases to substantially reduce the compression set of cured elastomers prepared using these bases. The bases comprise a gum type polydiorganosiloxane and a reinforcing filler that has been treated to reduce the occurrence of a phenomenon referred to as "creping" or "crepe hardening".
Compression set for elastomeric materials is defined as the residual deformation of the elastomer following removal of a compressive stress applied for a specified period of time. Compression set is measured thirty minutes after removal of the test specimen from a loading device in which the specimen had been subjected for a definite time to compression deformation under specified conditions of load application and environmental conditions. Compression set is an important property for many applications of elastomers, particularly in applications such as seals and gaskets.
The prior art describes various methods for processing curable organosiloxane compositions to reduce the compression set of cured elastomers prepared using these compositions. For example, U.S. Pat. No. 5,008,305, which issued on Apr. 16, 1991 describes treating a reinforcing silica with the combination of 1) an alkoxysilane containing one or two vinyl radicals per molecule and 2) an alkoxysilane containing 1 or 2 phenyl radicals per molecule.
U.S. Pat. No. 4,882,368, which issued on Nov. 21, 1989, teaches reacting a silanol-terminated polyorganosiloxane containing at least 80 mole percent of methyl-3,3,3-trifluoropropyl units with a silazane of the general formula X.sub.2 NH wherein X represents a silyl group selected from methylphenylvinylsilyl, ##STR1##
U.S. Pat. No. 4,785,047, which issued on Nov. 15, 1988 describes a method for preparing silica-filled liquid organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction to clear elastomers. The characterizing feature of the method is using a hexaorganodisilazane as at least a portion of an initial silica treating agent and following this with a second portion of disilazane that is equivalent to at least 5 weight percent of the disilazane present in the initial silica treating agent. Both portions of the disilazane are added during preparation of the curable organosiloxane composition.
U.S. Pat. No. 5,081,172, which issued on Jan. 14, 1992 to Chaffee et al. describes a method for reducing the compression set of a cured elastomer prepared from an organosiloxane elastomer base by blending the base with one of the ethylenically unsaturated disilazanes described in U.S. Pat. No. 4,882,368. The silica was treated with an organosilicon compound containing silanol or silicon-bonded hydrolyzable groups.
The ethylenically unsaturated disilazanes described in the Chaffee et al. patent react during curing of the base to form additional crosslinks. The resulting change in the physical properties of the cured elastomer may be undesirable for some end use applications.
Treatment of a reinforcing silica filler with ammonia followed by treatment with hexamethyldisilazane for the purpose of improving the storage stability of the composition and the physical properties of cured elastomers prepared from the composition is described in U.S. Pat. No. 3,635,743, that issued on Jan. 8, 1972.
The preparation of an organosiloxane elastomer base in the form of flowable powder is described in U.S. Pat. Nos. 3,824,208, which issued to Link and Scarbel on Jul. 16, 1974, commonly assigned British patent no. 2,147,598, and Japanese patent publication no. 2/102007, which was published on Apr. 13, 1990.
In accordance with the teaching of Link and Scarbel in the aforementioned patent, the finely divided base is prepared by first reducing the particle size of the polydiorganosiloxane to within the range of from 0.1 micron to 25.4 mm. by suitable means and then mixing the particles with at least 15 parts by weight of a reinforcing filler. This operation is conducted at a temperature of from 0 to 100 degrees C. preferably from 20 to 60 degrees C. A second critical process parameter in addition to the temperature is the speed of the tip of the blade in the mixer or other device used to subdivide the polymer and blend it with the filler during preparation of the powdered material. In accordance with the teaching of the patent, this speed can generally range from 50 to 150 feet per second and is preferably from 70 to 150 feet per second.
British patent no. 2,147,598, published on May 15, 1985 teaches a method for densifying the reinforcing and extending fillers used in organosiloxane compositions prepared using polyorganosiloxanes exhibiting viscosities from 1000 to 200,000,000 centipoise (1 to 200,000 Pa.s). The method comprises blending 100 parts by weight of the polydiorganosiloxane with from 50 to 500 parts by weight of a reinforcing filler or 400 to 5000 parts by weight of a non-reinforcing or extending filler. The vessel in which the blending is carried out is similar to the ones described in U.S. Pat. No. 3,824,208, with the exception that the mixing time in substantially longer in order to achieve the desired increase in particle density.
Japanese patent publication no. 2/102007 teaches blending a finely divided high consistency or "gel" type vinyl-containing polydiorganosiloxane having a viscosity of at least 1.times.10.sup.5 centipoise with a filler. A processing aid is included to prevent a phenomenon referred to as "creping" or "crepe hardening". The resultant mixture is then fed into a mixing apparatus equipped with a high speed rotating blade that is capable of subjecting the mixture to high speed mechanical shearing. During this step of the process the mixture achieves the consistency of a free-flowing powder. The critical parameters for obtaining a free flowing powder are the speed of the blade tip and the residence time of the mixture in the mixing apparatus.
U.S. Pat. No. 5,153,238, which issued on Oct. 4, 1992 teaches that formation of gel particles in finely divided organosiloxane elastomer bases can be avoided or substantially reduced by using a set of process conditions for preparing the bases that is beyond the limits and conditions described in the previous art. In addition to elimination of the gel particles the physical properties of cured elastomers prepared from the bases composition are substantially improved.
In accordance with the method described in U.S. Pat. No. 5,153,238, the reinforcing silica filler is maintained in a highly turbulent, fluidized state and at a temperature of from 100 to 200.degree. C. prior to addition of the polydiorganosiloxane, and the resultant mixture is subjected to shearing forces sufficient to rapidly reduce the ultimate particle size of the mixture to within the range of from 1 to 1000 microns. The entire process requires up to about one hour, depending upon the amount of filler and the desired particle size of the final product. The progress of the particle size reduction can be monitored by observing the electrical power consumed by the chopper blade(s) used to reduce the size of the ultimate particles and the blade(s) used to remove material from the wall of the mixing chamber and transport it to the chopper blade(s).
In attempting to reduce the compression set of a finely divided non-creping organosiloxane elastomer base by blending the base with various silica treating agents to further reduce the concentration of silanol groups in the base, the present inventor discovered that the reduction in compression set obtained using triorganosilanes containing a hydrolyzable group or hexaorganodisilazanes wherein the three hydrocarbon radicals bonded to each silicon atom are free of ethylenic unsaturation was at least as great as the decrease obtained using the ethylenically unsaturated hexaorganodisilazanes required by the prior art without substantially affecting other desirable physical properties of the cured elastomer. The present invention is based on this discovery.